Yes, using a longer stem on my road bike eased some back pain. So I expect, depending on relative sizes it could also cause it.
However you’ve changed a lot of variables: stem, bars and seat post. Going to a straight post will have decreased your reach effectively shortening your stem. Similarly wider bars will effectively lengthen your stem as you’re leaning forward more. I think I’ve got the effects right? But each change will affect your reach and position.
I’d change one thing at a time, get used to riding it, see if it’s comfortable and work from there? Ass you seem to want a dropper and Oder bars I’d keep that and switch stems around